Friction and wear of aluminium–steel contacts lubricated with ordered fluids-neutral and ionic liquid crystals as oil additives

Friction and wear of ASTM B211 aluminium–AISI 52100 steel contacts have been determined using pin-on-disk tests under variable conditions of normal applied load, sliding speed and temperature, in the presence of a lubricating base oil modified with a 1 wt.% proportion of three different liquid crystalline additives. The tribological behavior of the ionic liquid crystal n-dodecylammonium chloride (LC3) has been compared with that of two neutral liquid crystals: a non-polar species, 4,4′-dibutylazobenzene (LC1) which had previously shown its ability to lower friction and wear of metallic pairs as compared to the base oil, and a cholesterol derivative, cholesteryl linoleate (LC2). At low temperature and low sliding speed values, the friction coefficients obtained for LC1 are lower than those of LC3. As the severity of the contact conditions increases, this tendency reverses and the ionic species LC2 gives rise to lower friction values than LC1. Wear volume losses under increasing normal loads, between 2.45 and 5.89 N, are always lower in the presence of the ionic additive LC3. Lubrication and wear mechanisms are discussed from optical microscopy and SEM observation of the wear scars and wear debris morphology.